This heinous and misused phrase gets tossed around quite a bit by blood bankers. When we have patients with warm autoantibodies, it is usually impossible to find red cell units that are compatible in the crossmatch test. As a result, we may have to choose to give a product that is crossmatch-incompatible. When we do this, the blood bank worker most often tests the patient’s serum/plasma against multiple different red cell units, and chooses the one that shows the least amount of incompatibility (therefore, is “least incompatible” with the donor).
The problem is not only that the phrase is awkward (why on earth isn’t it “most compatible?”), but also that it doesn’t mean anything! There is no evidence, to my knowledge, that choosing red cell units that react less strongly with an autoantibody compared to other units improves post-transfusion survival of those red cells! Further, the term is often used as false reassurance to our transfusing staff (“Yes, doctor, the crossmatch is incompatible, but, by golly, we are going to give you the LEAST INCOMPATIBLE units!”). Those words may sound good, but they mean nothing to most of those ordering transfusions.
I don’t mind blood bankers using this silly phrase to each other, but I really do have a major problem with using it to reassure anyone outside of our blood banks. Further, I acknowledge that choosing a unit that reacts less strongly with patient autoantibodies over another that reacts more strongly is reasonable (though again, there’s no proof it is safer).
But don’t just take my word for it! The great Dr. Larry Petz, all the way back in 2003, argued that we should discard this phrase (or at least, to stop using it without a scientific definition). I can’t say it any better than he did in the last paragraph of the article linked above:
“Although least incompatible unit is a term that is entrenched in ‘in‐house’ jargon, it should not be used in discussions with attending physicians and, if used at all, should not be permitted outside the confines of the transfusion service. It is time for transfusion service personnel and clinicians to discuss issues surrounding transfusion of patients with AIHA in informative, scientific ways.” Lawrence D. Petz, MD, in Transfusion 2003
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